Phosphorescent materials are those that can absorb energy when excited by solar, fluorescent and other artificial lights. In particular, when these materials are excited by ultraviolet light, they deviate from their initial equilibrium states, and convert the absorbed energy to visible light after they return to the equilibrium states so as to continue to glow after the excitation has stopped. Material which can maintain a long glowing time is referred to as a "long afterglow phosphor".
Luminous paints applied on watch dials and safe marks have usually been formed by admixing phosphorescent material, using zinc sulfide mixed with copper (ZnS:Cu) as an activator, into paint, ink and the like. The sulfides can absorb energy when excited by ultraviolet light with certain wavelengths and then release the energy in the form of visible light in accordance with the above light-emitting principle of the phosphorescent material. Since the sulfides, however, have a very short span of afterglow, have unstable chemical structures and are not lightproof, there exist many problems when they are used in a practical situation. For example, the visible light emitted by such materials can last only for 20-30 minutes when they are used for luminous watches. There may also be the phenomenon of light decomposition and even loss of light-emitting ability of the material when it is radiated with ultraviolet radiation. Therefore, they can not be used in outdoor environments.
To prolong the time of afterglow, radioactive materials, such as Pm, are sometimes added to give phophors themselves the light-emitting ability. However, with radioactive materials the requirement for the treatment of the materials is very strict, and high costs arise for the aparatus used and the treatment of waste materials such as waste water, so this method is not used at present.
Besides sulfide phosphorescent material, it has been suggested that phosphorescent materials be prepared by adding the rare earth element europium to alkaline earth metal aluminates. For example, U.S. Pat. No. 3,294,699, to Lange, discloses a light-emitting material of strontium aluminate (SrAl.sub.2 O.sub.4 : Eu), in which divalent Eu is utilized as an activator and the amount added is 2-8 mol % of strontium aluminate. This fluorescent material has a light-emitting peak of 520 nm when excited by ultraviolet light. However, this fluorescent material has little afterglow.
Other examples of preparing fluorescent materials by adding other alkaline earth metals to strontium aluminate are disclosed in, for example, UK Patent No. 1,190,520, which discloses a divalent Eu-activated fluorescent material comprising (Ba)x, (Sr)y, (Ca)z, (Eu)p, Al.sub.12 O.sub.19 (wherein x+y+z+p=1, one or two of x, y and z may be 0,and 0.001.ltoreq.p.ltoreq.0.1). This sort of fluorescent material has a light-emitting peak of 380-440 nm wavelength when excited by ultraviolet. The material emits visible light only when excited by ultraviolet light or electrons, and is mainly used in CRT and the like.
Recently, other desirable light-emitting materials which can emit light for long periods when excited by a certain energy have been developed. For example, Chinese Patent Application No. CN 1,053,807A discloses a light-emitting material of long afterglow, which has a general formula of M{Sr(1-x)Eu x}O.multidot.nAl.sub.2 O.sub.3 .multidot.yB.sub.2 O.sub.3 (1.ltoreq.m.ltoreq.5, 1.ltoreq.n.ltoreq.8, and 0.001.ltoreq.y.ltoreq.0.35). This long afterglow light-emitting material is formed by using oxide of aluminum, strontium and europium, or salts which can produce these oxides when heated, as starting materials, sintering at 1200.degree.-1600.degree. C., and reducing at 1000.degree.-1400.degree. C. under N.sub.2 and H.sub.2. However, the actual time of afterglow of this light-emitting material of long afterglow is only 4-5 hours, and it has a low initial brightness and poor applicablity.
To solve the above-described problems, the inventors of the present invention have improved the light-emitting combination of activators of alkaline earth metal aluminates and rare earth metals, studied various structures and new crystals, and developed a phosphor which exibits higher initial brightness and longer afterglow than the light-emitting materials of combination of known sulfides and activators of alkaline earth metal aluminates and rare earth metals.